1. Field of the Invention
The present invention relates in general to reduction of stray electromagnetic radiation emissions produced by electronic systems and more specifically to apparatus for shielding stray electromagnetic radiation.
2. Background of the Invention
Electronic systems (e.g., electrical circuits, etc.) which operate on alternating current (xe2x80x9cACxe2x80x9d) or direct current (xe2x80x9cDCxe2x80x9d) voltage generate an electromagnetic field. An electromagnetic field is comprised of waves. Such fields are created because of the electrical energy being used in the devices. In most countries, agencies like the Federal Communications Commission (FCC) in the United States, Industry Canada (ICAN) in Canada, and Bundesamt Fur Post Und Telekommunikation-Zulassen Und Testen (BZT) in Germany regulate maximum amplitudes for radiated and conducted electromagnetic energy permissible at specific frequencies.
Unintentional radiators include electronic systems such as personal computers and computer networking hardware. Intentional radiators include electronic systems such as transmitters and regenerative receivers. This is by no means an exhaustive list of unintentional and intentional radiators, as various other electronic systems produce unwanted stray electromagnetic radiation emissions. Such unwanted electromagnetic radiation includes radio frequency, electrostatic and magnetic fields. As the operating speeds of these electronic systems increase, the strength of the electromagnetic fields increase. For example, present personal computers can operate at speeds above two hundred megahertz (xe2x80x9cMHzxe2x80x9d). These operating speeds will only increase in the future.
Stray electromagnetic fields are highly undesirable, as they can cause interference with other electronic devices, radio frequency communications systems, etc. Because of this, governmental agencies of many agencies have issued regulations which specify the maximum amplitude of stray unwanted electromagnetic radiation that an electronic system can emit and still be used in that country. If a product does not meet these specifications, it cannot be sold in that country.
Each of the government agencies has specific testing methodologies. In general, the stray electromagnetic radiation of an electronic system is measured using an antenna placed at least three meters from an electronic system on a calibrated test site. The field strength per meter of the stray fields is measured. In general, stray fields have strength with an order of magnitude in the one hundred microvolts range when the frequency is between thirty MHz and eighty-eight MHz. When recorded field strength exceeds the limit imposed by agencies such as the FCC, the electronic system must be redesigned so that it complies with those specifications. Electronic system redesign and implementation into production can delay product introduction by several months.
At present, a common method to reduce the stray electromagnetic energy produced by an electronic system is to encase the electronic system in a continuous metal enclosure. If there are no seams in an enclosure, then electromagnetic energy is contained since a Faraday Cage has essentially been built around the electromagnetic energy source. When designing such an enclosure, the amplitude and frequency of the stray electromagnetic field determines its thickness. Regardless of the thickness, however, the shielding properties of the enclosure are compromised if system being shielded has any input/output ports, data cables, displays, etc. that cannot be shielded. Because of this, additional shielding must usually be added to the enclosures, cables, displays, keyboards, etc. to assure compliance to the FCC recommendations. These additional technologies add significant product cost and development time, and detract from the cosmetics of a final product.
Summarizing, while effective, these enclosures are heavy, add cost, and reduce the appeal of the product, especially if it is intended for the consumer market. Furthermore, it is entirely possible that this shielding might not even be necessary. Thus, a product could be designed with such shielding, thereby greatly adding to its cost, when the shielding was not even necessary.
In addition, in an attempt to make products that are more attractive and/or less expensive, the electronics industry also uses plastic enclosures. If an electronic system operating inside a plastic enclosure exceeds electromagnetic interference limits allowed by the regulatory agency, a metallized surface may be applied to the plastic enclosure in attempt to transform the interior of the plastic enclosure into a metal enclosure. The cost of this technology can be prohibitive, however, because the top and bottom sections of the enclosure must exhibit electrical continuity at the seams, while maintaining a uniform thickness over the entire enclosure surface. This is difficult to achieve.
Another common method for reducing stray electromagnetic radiation is to change the location of components in an electronic system. For example, a designer might change the location of the system""s crystal oscillator to place it closer to the circuit receiving the signal. This will reduce the length of the signal path (i.e., the connecting wire or printed circuit board trace) and therefore might reduce the stray electromagnetic radiation emitted. The problem with this particular method is that it requires a redesign of the electronic system. This increases the cost of the product and significantly delays the product from entering the market. In addition, there is no way of knowing if the redesign actually worked until it is retested. When it is retested, the redesigned product may still exceed the specifications for maximum stray electromagnetic radiation. Many times, a third redesign must be undertaken. This costs time and money.
Examples of the specifications for the maximum stray electromagnetic radiation that a product must satisfy to be sold in the United States include:
These specifications, which have been issued by the FCC as of the filing date of this application, are measured from a position three meters from the electronic system undergoing test.
Examples of specifications that are in force in Europe include:
These are also measured at a distance of three meters from the electronic system undergoing test. These three meter limits are mandated in the specification EW 55022/CISPR 22, which is entitled xe2x80x9cLimits and Methods of Measurement of Radio Disturbance Characteristics of Information Technology Equipment.xe2x80x9d
Furthermore, according to the newest European criteria, many products must continue to operate and be less susceptible to a field of three volts per meter from twenty-seven MHz to five hundred MHz to be allowed entry into the marketplace for sale. This is known as susceptibility. These criteria for susceptibility is best represented by the following product standards:
IEC 801-2 Electromagnetic Compatibility for Industrial-Process Measurement and Control Equipment. Part 2: Electrostatic Discharge Requirements. 1984
IEC 801-3 Electromagnetic Compatibility for Industrial Process Measurement and Control Equipment. Part 3: Radiated Electromagnetic Field Requirements. 1984
IEC 801-4 Electromagnetic Compatibility for Industrial Process Measurement and Control Equipment. Part 4: Electrical Fast Transient. 1988.
An example of an electronic system that is particularly prone to producing stray fields is a laptop computer system. Prior to being marketed for sale or being sold, a laptop computer system must be tested by a FCC registered testing laboratory. The product must comply with the Code of Federal Regulations, Title 47, Part 2 entitled Frequency Allocations and Radio Treaty Matters; General Rules and Regulations; and Part 15 entitled Radio Frequency Devices, Oct. 1, 1996 edition. If the product does not pass this testing, it cannot be marketed for sale or sold. Laptop computer systems represent a fast paced electronic system technology that must be introduced into the market quickly. If the laptop computer system fails the testing described above, it must be redesigned prior to sale, thereby resulting in delay. Delay in releasing the product to the market can result in obsolescence before the first unit is sold. The testing and redesign stage of product compliance can sometimes be the critical path for product or company success
Compounding the above-described problems is that when designing electronic systems, it is difficult to predict the type and amount of stray fields that will be created; Because of this, designers either design their product without these radiation reducing features and accept the risk of product delays should the product fail testing, or implement potentially unnecessary field control structures (e.g., a metal enclosure). However, product delays can be fatal for many products, as time-to-market is extremely important in the modem marketplace. This is especially true in the electronics market. For example, technology and consumer tastes for personal computers generally change within a matter of months. If a product must be redesigned before it can be sold, it probably will never be sold. On the other hand, unnecessary field control structures increase the product""s cost, thereby increasing the chance for the product""s failure in the marketplace.
Thus, there has been a long felt need for an inexpensive apparatus that reduces stray electromagnetic radiation of products without requiring a product redesign or heavy shielding.
The present invention overcomes the problems and disadvantages of the prior art through a unique electromagnetic radiation shield. A shield or tab is disclosed. The shield of the present invention comprises a first conductive layer and a second conductive layer separated by an insulator. In a preferred embodiment, the first conductive layer and second conductive layer comprise a flexible aluminum alloy. The insulating layer of the preferred embodiment comprises a fibrous material that is bonded to the first conductive layer and second conductive layer by paraffin-based glue. The second conductive layer has an adhesive on the exterior thereof for affixing the shield or tab to a component that emits unwanted electromagnetic radiation. If desired, the first conductive layer can have a material applied to the exterior thereof that allows printing images thereon.
The various embodiments of the present invention are installed on the surface of electronic devices that are emitting unwanted electromagnetic radiation. If during testing, a product emits unacceptable amplitudes and/or frequencies of eletromagnetic radiation, measurements can be taken in the vicinity of the electronic components installed therein. The shields of the present invention can be installed on the surface of selected electronic components of the apparatus in the field. Then the product can be immediately retested.
Selection of the electronic components that will have the shields of the present invention installed therein varies from apparatus to apparatus. In some applications, the shields can be placed only on those components exhibiting unacceptable electromagnetic radiation performance. In other cases, the shields of the present invention will be placed on several components of the apparatus.
The above and other preferred features of the invention, including various novel details of implementation and combination of elements will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular methods and circuits embodying the invention are shown by way of illustration only and not as limitations of the invention. As will be understood by those skilled in the art, the principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.